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The invention relates generally to the field of image processing, and in particular to embedding and extracting hidden messages in digital image data. This field is also referred to as data hiding, information hiding, watermarking and also steganography.
U.S. Pat. No. 6,044,156, issued Mar. 28, 2000, entitled xe2x80x9cMethod For Generating An Improved Carrier For Use In An Image Data Embedding Application,xe2x80x9d by Honsinger et al., discloses a technique for embedding messages in digital images data. This method for embedding a hidden message into a digitized image includes the steps of providing a message template indicating the relative locations of data in the embedded message, the relative locations of the data being such that the autocorrelation of the message template is strongly peaked; placing message data at the data locations defined by the template; convolving the message data in the template with a carrier to form a dispersed message; and combining the dispersed message with the image.
The hidden messages are recovered from the embedded hidden message image by cross correlating the embedded hidden message image containing the dispersed message with a decoding carrier to recover the embedded dispersed message; and extracting the digital message data from the recovered dispersed message employing the message template to locate the message data.
One problem with the above described approach is that when the digital image is very noisy, for example when it is formed by scanning an inkjet printed image on ordinary paper, the method sometimes does not work to recover the embedded message. There is a need therefore for an improved method of recovering the embedded message.
The problem is solved according to the present invention by providing a method for extracting an embedded message from a digital image, the embedded message being formed by arranging message bits in predetermined locations represented by a message template, convolving the message with a random phase, flat Fourier amplitude carrier to form a dispersed message and tiling the dispersed message over the image, that includes the steps of:
a) locating the tile boundaries in the digital image;
b) correlating a tile with the carrier to extract the embedded message;
c) forming a multiplier A that is inversely proportional to the noise in the extracted message;
d) multiplying the embedded message by A to form a weighted embedded message;
e) repeat steps b) to d) for a second tile to generate a second weighted embedded message;
f) summing the weighted embedded messages to form a summed weighted embedded message; and
h) extracting the message bits from the summed weighted embedded message.
The method has the advantage of improving the signal quality for data embedding applications by weighting each recovered embedded message inversely with respect to noise in the recovered message and adding the recovered messages to reinforce the signal and cancel the noise. The improvement due to the method can be significant for images with large variations in business, such as portraits, or in images that are very noisy, such as digital images produced by scanning ink jet prints on plain paper.